Platinum(II) 2,4-Di(2′-pyridyl)-6-(p-tolyl)-1,3,5-triazine complexes: Synthesis and photophysics

Article abstract of DOI:10.1021/om701285c

Five platinum(II) complexes based on the 2,4-di(2′-pyridyl)-6-(p- tolyl)-1,3,5-triazine ligand (1-5) have been synthesized and characterized. The effects of the triazine-containing terdentate ligand on the photophysics and nonlinear transmission have been explored. The influence of the monodentate ligand, i.e. chloride, pentynyl, and phenylacetylide, and counteranion on the photophysical and nonlinear transmission characteristics of these complexes has also been systematically investigated. It is found that, due to the electron-deficient nature of the triazine ring and the extension of the π system between the terdentate ligand and the 6-(p-tolyl) substituent, the metal-to-ligand charge transfer (MLCT) bands in both the UV-vis spectra ( ¹MLCT) and the emission spectra (³MLCT) red-shift in comparison to those of the corresponding platinum(II) complexes with 2,2′:6′,2″-terpyridyl ligands. However, both the emission efficiency from the ³MLCT state and the emission lifetime are significantly reduced. The different monodentate ligands cause a decrease of the energy of the ¹MLCT band, going down from the complex with a chloride ligand (1 and 2) to 3 and 4 with a pentynyl ligand to 5 with a phenylacetylide ligand. The chloride-containing complex (2) only shows a terdentate ligand ¹ππ* originated emission at 460 nm, while the pentynyl-containing complexes (3 and 4) exhibit an ³MLCT-based emission at ～615 nm with a lifetime of approximately 180 ns. In contrast, complex 5 with the phenylacetylide ligand emits at both 460 and 622 nm. Among these five complexes, only 3 and 4 exhibit a moderate triplet transient absorption, with a positive band extending from 510 to 820 nm. The lifetimes of the transients are approximately 200 ns, which is tentatively assigned to the ³MLCT excited state. Complexes 3-5 display a weak nonlinear transmission that is affected by the monodentate ligand and the counteranions. In addition, it is revealed that the counteranion has a negligible effect on the ¹MLCT state, whereas its effect on the ³MLCT state is drastic, especially on the triplet excited-state absorption coefficient, the quantum yield of the triplet state formation, the quantum yield of the ³MLCT emission, and the nonlinear transmission, as exemplified by complexes 3 and 4.

Full text of DOI:10.1021/om701285c

Organometallics 2008, 27, 2743–2749
2743
Platinum(II) 2,4-Di(2′-pyridyl)-6-(p-tolyl)-1,3,5-triazine Complexes:
Synthesis and Photophysics
Pin Shao, Yunjing Li, and Wenfang Sun*
Department of Chemistry and Molecular Biology, North Dakota State UniVersity,
Fargo, North Dakota 58105-5516
ReceiVed December 23, 2007
Five platinum(II) complexes based on the 2,4-di(2′-pyridyl)-6-(p-tolyl)-1,3,5-triazine ligand (1-5) have
been synthesized and characterized. The effects of the triazine-containing terdentate ligand on the
photophysics and nonlinear transmission have been explored. The influence of the monodentate ligand,
i.e. chloride, pentynyl, and phenylacetylide, and counteranion on the photophysical and nonlinear
transmission characteristics of these complexes has also been systematically investigated. It is found
that, due to the electron-deficient nature of the triazine ring and the extension of the π system between
the terdentate ligand and the 6-(p-tolyl) substituent, the metal-to-ligand charge transfer (MLCT) bands in
both the UV-vis spectra (1MLCT) and the emission spectra (3MLCT) red-shift in comparison to those
of the corresponding platinum(II) complexes with 2,2′:6′,2′′-terpyridyl ligands. However, both the emission
3
efficiency from the MLCT state and the emission lifetime are significantly reduced. The different
monodentate ligands cause a decrease of the energy of the 1MLCT band, going down from the complex
with a chloride ligand (1 and 2) to 3 and 4 with a pentynyl ligand to 5 with a phenylacetylide ligand. The
1
chloride-containing complex (2) only shows a terdentate ligand π,π* originated emission at 460 nm,
while the pentynyl-containing complexes (3 and 4) exhibit an 3MLCT-based emission at ∼615 nm with
a lifetime of approximately 180 ns. In contrast, complex 5 with the phenylacetylide ligand emits at both
460 and 622 nm. Among these five complexes, only 3 and 4 exhibit a moderate triplet transient absorption,
with a positive band extending from 510 to 820 nm. The lifetimes of the transients are approximately
200 ns, which is tentatively assigned to the 3MLCT excited state. Complexes 3-5 display a weak nonlinear
transmission that is affected by the monodentate ligand and the counteranions. In addition, it is revealed
that the counteranion has a negligible effect on the 1MLCT state, whereas its effect on the 3MLCT state
is drastic, especially on the triplet excited-state absorption coefficient, the quantum yield of the triplet
state formation, the quantum yield of the 3MLCT emission, and the nonlinear transmission, as exemplified
by complexes 3 and 4.
and protein probes,4 and dye-sensitized solar cells.5 All these
applications are based on their square-planar configuration and
unique spectroscopic properties. Photophysical studies have
revealed that the nature of the low-lying charge transfer (CT)
band of the platinum terpyridyl complexes has a great effect
on their optical properties.6 Recent theoretical studies on some
amino-containing platinum(II) terpyridyl complexes indicated
that the low-lying CT band could be ascribed to a metal-to-
ligand charge-transfer (MLCT) transition mixed with some
ligand-to-ligand charge-transfer (LLCT) transition.7 In both
cases, the lowest unoccupied molecular orbital (LUMO) is
Introduction
Platinum(II) terpyridyl complexes have attracted a great deal
of interest in recent years due to their versatile potential
applications in chemical sensors,1 organic light-emitting devices
(OLED),2 nonlinear transmission devices,3 DNA intercalation
* To whom correspondence should be addressed. E-mail:
Wenfang.Sun@ndsu.edu.
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L.-P.; Tung, Z.-H. Inorg. Chem. 2004, 43, 5195. (b) Wu, L. Z.; Cheung,
T. C.; Che, C. M.; Cheung, K. K.; Lam, M. H. W. Chem. Commun. 1998,
10, 1127. (c) Wong, K.-H.; Chan, M. C.-W.; Che, C. M. Chem. Eur. J.
1999, 5, 2845. (d) Kui, S. C. F.; Chui, S. S.-Y.; Che, C.-M.; Zhu, N. J. Am.
Chem. Soc. 2006, 128, 8297.
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Lin, Z. Inorg. Chem. 2006, 45, 10922.
(5) (a) Wadas, T. J.; Chakraborty, S.; Lachicotte, R. J.; Wang, Q.-M.;
Eisenberg, R. Inorg. Chem. 2005, 44, 2628. (b) Chakraborty, S.; Wadas,
T. J.; Hester, H.; Flaschenreim, C.; Schmehl, R.; Eisenberg, R. Inorg. Chem.
2005, 44, 6865.
(6) (a) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.;
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2001, 40, 2193. (b) Crites, D. K.; Cunningham, C. T.; McMilin, D. R. Inorg.
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Phys. Chem. A 2005, 109, 8809. (b) Liu, X.-J.; Feng, J.-K.; Meng, J.; Pan,
Q.-J.; Ren, A.-M.; Zho, X.; Zhang, H.-X. Eur. J. Inorg. Chem. 2005, 1856.
(c) Hua, F.; Kinayyigit, S.; Cable, J. R.; Castellano, F. N. Inorg. Chem.
2006, 45, 4304.
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Phys. Lett. 2003, 82, 850. (b) Guo, F.; Sun, W.; Liu, Y.; Schanze, K. Inorg.
Chem. 2005, 44, 4055. (c) Sun, W.; Guo, F. Chin. Opt. Lett. 2005, S3,
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10.1021/om701285c CCC: $40.75
2008 American Chemical Society
Publication on Web 05/20/2008

2744 Organometallics, Vol. 27, No. 12, 2008
Shao et al.
starting chemicals were purchased from Alfa Aesar, and the solvents
were used as received unless otherwise stated.
1H NMR spectra were measured on a Varian 300 MHz VNMR
spectrometer. ESI-HRMS analyses were conducted on a Bruker
Daltonics BioTOF III mass spectrometer. Elemental analyses were
conducted by NuMega Resonance Laboratories, Inc., San Diego,
CA.
Complex 1. A 0.163 g portion of L and 0.208 g of K2PtCl4
were refluxed in 30 mL of acetonitrile/H2O (1/1, v/v) for 24 h.
The volume of the reaction mixture was then reduced to ∼10 mL,
and the precipitates were collected. The solid was dissolved in a
small amount of DMF, and this solution was filtered into a saturated
NH4PF6 aqueous solution. The mixture was then stirred overnight
at room temperature. The yellow-brown precipitates were filtered
out, washed with water and ether, and dried in a vacuum oven.
Figure 1. Structures of platinum(II) 2,4-di-2′-pyridyl-6-p-tolyl-
1,3,5-triazine complexes.
located on the terpyridyl ring. Thus, both the absorption and
the emission bands associated with the CT transition(s) could
be tuned by modification of the terpyridyl ligand.3d,6,8 In general,
introducing electron-withdrawing substituents (-CN, -NO2,
etc.) on the 4′-position of the terpyridyl ligand or extending the
conjugation of the terpyridyl ligand by incorporating a 4′-aryl
substituent on the terpyridyl ligand can stabilize the LUMO.6
As a result, the low-lying CT band(s) will red-shift. However,
the effect of expanding the conjugation by the 4′-aryl substit-
uent(s) is usually limited, because the unfavorable repulsive
interactions between the hydrogens adjacent to the interannular
bond cause the 4′-aryl substituent to twist out of the terpyridyl
plane, which reduces the π-orbital overlap between the aryl
substituent and the terpyridyl rings. To remove the unfavorable
repulsive interactions, Hanan and co-workers replaced the central
pyridine ring with a triazine ring or used the 4′-(2-pyrimidyl)
substituent instead of the 4′-phenyl substituent and investigated
the photophysics of the corresponding Ru(II) and Zn(II)
complexes.9 The crystal structures of the Ru complexes have
revealed that the 4′-aryl substituent lies almost coplanar with
the terdentate ligand.9a–d The MLCT band energy is thus
reduced, showing a red shift in the absorption and the emission
spectra and an increased emission lifetime. In addition, Fe, Co,
Ni, and Cu complexes based on the 2,4-di(2′-pyridyl)-1,3,5-
triazine ligand have also been reported by this group.10 However,
to date, no platinum(II) complexes have been reported based
on this approach. To remedy this deficiency, our group recently
synthesized a series of platinum(II) complexes (Figure 1)
containing the 2,4-di(2′-pyridyl)-6-(p-tolyl)-1,3,5-triazine ligand
(L) and investigated their photophysical properties and nonlinear
transmission behavior. Different monodentate ligands (chloride,
pentynyl, and phenylacetylide) and different counteranions
1
Yield: 41%. H NMR (DMSO-d6): δ 9.14 (d, J ) 4.5 Hz, 2H),
8.94 (d, J ) 7.5 Hz, 2H), 8.76 (d, J ) 8.1 Hz, 2H), 8.70 (td, J )
1.5 and 7.8 Hz, 2H), 8.25 (t, J ) 6.6 Hz, 2H), 7.57 (d, J ) 8.4 Hz,
2H), 2.48 (s, 3H) ppm. ESI-MS: m/z calcd for [C20ClH15N5Pt194 +
]
555.0660, found 554.9987 (97%); m/z calcd for [C20ClH15
N5Pt195 +
556.0653, found 555.9980 (100%); m/z calcd for
[C20ClH15N5Pt
196 + 557.0652, found 556.9972 (47%). Anal. Calcd
-
]
]
for C20H15ClF6N5PPt: C, 34.27; H, 2.16; N, 9.99. Found: C, 34.27;
H, 1.96; N, 10.23.
Complex 2. The procedure was similar to that for the synthesis
of 1, except Mg(ClO4)2 was used instead of NH4PF6. Yield: 46%.
1H NMR (DMSO-d6): δ 9.11 (d, J ) 5.4 Hz, 2H), 8.94 (d, J ) 7.5
Hz, 2H), 8.76 (d, J ) 8.1 Hz, 2H), 8.70 (t, J ) 8.1 Hz, 2H), 8.23
(t, J ) 6.6 Hz, 2H), 7.56 (d, J ) 8.7 Hz, 2H), 2.49 (s, 3H) ppm.
ESI-MS: m/z calcd for [C20ClH15N5Pt194 +
]
555.0660, found
195 + 556.0653, found
556.1108 (100%); m/z calcd for [C20ClH15N5Pt196 +
557.0652,
555.1128 (94%); m/z calcd for [C20ClH15N5Pt
]
]
found 557.1110 (46%). Anal. Calcd for C20H15Cl2N5O4Pt: C, 36.65;
H, 2.31; N, 10.69. Found: C, 36.91; H, 2.70; N, 10.89.
Complex 3. A mixture of 30 mg of KOH in 200 mL of methanol
was purged with argon for 30 min. Then 40 µL of pentyne was
added, and the mixture was stirred for 30 min under an argon
atmosphere. A 100 mg portion of 1 and 5 mg of CuI were added.
The mixture was continuously stirred under argon at room tem-
perature for 48 h. After that, the solvent was removed under reduced
pressure. The residue was extracted with dichloromethane and dried
over anhydrous Na2SO4. The crude product was purified by passing
through a short silica column twice; dichloromethane/acetone (20/
1, v/v) was used as the eluent. The product was further purified by
1
-
(ClO4 and PF6-) are used to study the effect of the mono-
recrystallization from dichloromethane and ether. Yield: 2%. H
NMR (CD3CN): δ 9.08 (d, J ) 4.5 Hz, 2H), 8.74 (m, 4H), 8.51 (t,
J ) 8.1 Hz, 2H), 7.94 (br., s, 2H), 7.56 (d, J ) 7.5 Hz, 2H), 2.54
(s, 3H), 2.48 (t, J ) 7.5 Hz, 2H), 1.59 (q, J ) 7.2 Hz, 2H), 1.02
dentate ligand and the counteranion on the photophysics of these
platinum(II) complexes.
(t, J ) 7.2 Hz, 3H) ppm. ESI-MS: m/z calcd for [C25H22N5Pt194 +
]
]
]
586.1496, found 586.0775 (74%); m/z calcd for [C25H22N5Pt195 +
587.1519, found 587.0801 (100%); m/z calcd for [C25H22N5Pt196 +
588.1528, found 588.0822 (84%). Anal. Calcd for C25H22
Experimental Section
Synthesis. The ligand 2,4-di(2′-pyridyl)-6-(p-tolyl)-1,3,5-triazine
(L) was synthesized according to the reported procedure.9b All the
-
F6N5PPt · 1.8CH2Cl2: C, 36.36; H, 2.91; N, 7.91. Found: C, 36.47;
H, 3.38; N, 7.55.
(8) (a) Michalec, J. F.; Bejune, S. A.; McMillin, D. R. Inorg. Chem.
2000, 39, 2708. (b) Chakraborty, S.; Wadas, T. J.; Hester, H.; Flaschenreim,
C.; Schmehl, R.; Eisenberg, R. Inorg. Chem. 2005, 44, 6284. (c) Crites,
D. K.; Cunningham, C. T.; McMillin, D. R. Inorg. Chim. Acta 1998, 273,
346.
Complex 4. The procedure was similar to that for the synthesis
of 3, except 2 was used instead of 1. Yield: 7%. 1H NMR (CD3CN):
δ 9.18 (br s, 2H), 8.70 (m, 4H), 8.51 (br s, 2H), 7.99 (br s, 2H),
7.53 (d, J ) 6.9 Hz, 2H), 2.53 (s, 3H), 2.46 (m, 2H), 1.65 (m, 2H),
(9) (a) Polson, M. I. J.; Taylor, N. J.; Hanan, G. S. Chem. Commun.
2002, 1356. (b) Polson, M. I. J.; Medlycott, E. A.; Hanan, G. S.; Mikelsons,
L.; Taylor, N. J.; Watanabe, M.; Tanaka, Y.; Loiseau, F.; Passalacqua, R.;
Campagna, S. Chem. Eur. J. 2004, 10, 3640. (c) Medlycott, E. A.; Hanan,
G. S. Inorg. Chem. Commun. 2007, 10, 229. (d) Fang, Y.-Q.; Taylor, N. J.;
Laverdiere, F.; Hanan, G. S.; Loiseau, F.; Nastasi, F.; Campagna, S.;
Nierengarten, H.; Leize-Wagner, E.; Dorsselaer, A. V. Inorg. Chem. 2007,
46, 2854. (e) Medlycott, E. A.; Hanan, G. S. Coord. Chem. ReV. 2006,
250, 1763.
1.09 (t, J ) 7.8 Hz, 3H) ppm. ESI-MS: m/z calcd for [C25
H22N5Pt194 +
586.1496, found 586.1909 (76%); m/z calcd for
[C25H22N5Pt195 +
587.1519, found 587.1937 (100%); m/z calcd
for [C25H22N5Pt
196 + 588.1528, found 588.1961 (81%). Anal. Calcd
-
]
]
]
for C25H22N5ClO4Pt · 0.8CH2Cl2: C, 41.05; H, 3.15; N, 9.28. Found:
C, 40.99; H, 3.23; N, 8.96.
Complex 5. A mixture of 30 mg of KOH in 5 mL of DMF was
purged with argon for 30 min, and then 20 µL of phenylacetylene
was added and the mixture was stirred for 30 min. A 120 mg portion
(10) Medlycott, E. A.; Udachin, K. A.; Hanan, G. S. Dalton Trans. 2007,
430.

Platinum(II) 1,3,5-Triazine Complexes
Organometallics, Vol. 27, No. 12, 2008 2745
of 1 and 8 mg of CuI were added. After the mixture was stirred
under argon at room temperature for 24 h, the reaction mixture
was filtered and 50 mL of ether was added to the filtrate. The
precipitates were then collected and purified twice by recrystalli-
zation in acetonitrile and ether. A brown-red solid was obtained
1
(yield 57%). H NMR (DMSO-d6): δ 9.22 (s, 2H), 8.84 (d, J )
7.2 Hz, 2H), 8.64 (m, 4H), 8.12 (s, 2H), 7.49 (t, J ) 9.2 Hz, 4H),
7.31 (t, J ) 7.2 Hz, 2H), 7.23 (t, J ) 7.2 Hz, 1H), 2.46 (s, 3H)
ppm. ESI-MS: m/z calcd for [C28H20N5Pt194 +
]
620.1340, found
620.2302 (75%); m/z calcd for [C28H20N5Pt195 +
621.1363,
found 621.2326 (100%); m/z calcd for [C28H20N5Pt
196 + 622.1373,
]
]
found 622.2340 (86%). Anal. Calcd for C28H20N5PtPF6: C, 43.87;
H, 2.63; N, 9.14. Found: C, 43.84; H, 2.65; N, 9.54.
Photophysical Measurements. The UV-vis absorption spectra
were measured on an Agilent 8453 spectrophotometer. Dichlo-
romethane was used as the solvent. The steady-state emission
spectra were obtained on a SPEX Fluorolog-3 fluorometer/
phosphorometer. The emission quantum yields of the samples were
determined by a comparative method,11 in which a degassed
aqueous solution of [Ru(bpy)3]Cl2 (Φem ) 0.042, excited at 436
nm)12 was used as the reference. The emission lifetimes and the
triplet transient difference absorption spectra were measured on an
Edinburgh LP920 laser flash photolysis spectrometer. The third
harmonic output (355 nm) of a Nd:YAG laser (Quantel Brilliant,
pulse width (fwhm) ∼4.1 ns, repetition rate 10 Hz) was used as
the excitation source. The laser repetition rate was set to 1 Hz, and
the output energy at 355 nm was adjusted to ∼5 mJ/pulse. Each
sample was purged with argon for 30 min before measurement.
The triplet excited-state molar extinction coefficient and triplet
quantum yield were determined by the partial saturation method.13
The optical density at 705 nm was monitored when the excitation
energy at 355 nm was gradually increased. Saturation was observed
when the excitation energy was higher than 3.6 mJ for 3 and 5.5
mJ for 4. The following equation was then used to fit the
experimental data to obtain εT and ΦT:13
Figure 2. Electronic absorption spectra of complexes 1-5 in
CH2Cl2.
metal-to-ligand charge transfer (1MLCT) transitions, which have
been reported in many other platinum(II) terpyridyl complexes
(Table 1).3b,6,7,14 For complexes 1 and 2, the MLCT band
maximum is located at ∼420 nm, with a shoulder appearing at
∼450 nm. In contrast, the MLCT bands of their corresponding
acetylide complexes 3-5 become broader and red-shift to 450
nm. Especially for complex 5, there is an additional shoulder
appearing at ca. 512 nm, which indicates that there might be
two orbitally distinct MLCT transitions, e.g., dxz(Pt) f π*(L)
and dyz(Pt) f π*(L) (assuming that the acetylide ligand is along
the x axis with z perpendicular to the coordination plane). This
is consistent with what has been observed in platinum(II)
terpyridyl acetylide complexes1a,3b and diimine platinum(II) bis-
1
acetylide complexes.15 The red shift of the MLCT band in
complexes 3-5 in comparison to the bands in 1 and 2 can be
explained by the fact that the HOMOs of the platinum
complexes are predominantly metal-based and the energy of
the HOMOs could be modulated by the auxiliary ligand(s). The
strong electron-donating ability of the acetylide ligand(s) would
destabilize the HOMO(s), thus leading to a decrease of the
energy gap between the metal-based HOMO and the terdentate
ligand-based LUMO. On comparison of the absorption spectra
of the platinum 4′-tolylterpyridyl analogues3b to those of
∆OD ) R(1 - exp(-bIp))
(1)
where ∆OD is the optical density change at 705 nm, Ip is the pump
intensity in einstein cm-2, a ) (εT - ε0)dl, and b ) 2303ε0exΦT/
A. εT and ε0 are the absorption coefficients of the excited state and
ex
the ground state at 705 nm, ε0 is the ground-state absorption
coefficient at the excitation wavelength of 355 nm, d is the
concentration of the sample (mol L-1), l is the thickness of the
sample excited by the excitation beam (namely the diameter of
the excitation beam), and A is the area of the sample irradiated by
the excitation beam. The concentration used for this experiment
was 3.0 × 10-5 mol L-1 for 3 and 2.9 × 10-5 mol L-1 for 4.
Nonlinear Transmission Measurements. The nonlinear trans-
mission measurements were conducted using a setup that has been
described previously.3b,c A Quantel Brilliant Nd:YAG laser was
used as the light source and was operated at its second-harmonic
output (532 nm) with a 10 Hz repetition rate.
1
complexes 1-5, it is quite obvious that the MLCT bands of
1-5 are broader and more red-shifted. For example, the 1MLCT
band of 4 extends to 550 nm, while its 4′-tolylterpyridyl
analogue shows almost no absorption beyond 500 nm.16
Complex 5 exhibits a moderate absorption from 500 to 600 nm,
while its 4′-tolylterpyridyl analogue is transparent above 550
nm.3b This red shift can be attributed to the following two
factors: (1) the π* orbital of the triazine-containing terdentate
ligand is lower than that of the 4′-tolylterpyridyl ligand due to
the electron-deficient nature of the triazine ring, and (2) the
unfavorable repulsive interactions between the hydrogens
adjacent to the interannular bond have been removed in the
triazine-based ligand, which yields an almost coplanar config-
uration between the tolyl substituent and the terdentate ligand.
Results and Discussion
Electronic Absorption Spectra. Figure 2 shows the elec-
tronic absorption spectra of complexes 1-5 in dichloromethane
(CH2Cl2). All these complexes exhibit strong absorbing bands
below 400 nm, which could be assigned to the 1π,π* transitions
within the 2,4-di(2′-pyridyl)-6-(p-tolyl)-1,3,5-triazine ligand. The
broad bands from 400 to 600 nm most likely arise from the
(14) (a) Wong, K. M.-C.; Tang, W.-S.; Chu, B. W.-K.; Zhu, N.; Yam,
V. W.-W. Organometallics 2004, 23, 3459. (b) Shikhova, E.; Danilov, E. O.;
Kinayyigit, S.; Pomestchenko, I. E.; Tregubov, A. D.; Camerel, F.; Retaileau,
P.; Ziessel, R.; Castellano, F. N. Inorg. Chem. 2007, 46, 3038. (c) Yam,
V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K. Organometallics
2001, 20, 4476.
(11) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(12) van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 4853.
(13) (a) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15,
1. (b) Schermann, G.; Schmidt, R.; Vo¨lcker, A.; Brauer, H.-D.; Mertes, H.;
Franck, B. Photochem. Photobiol. 1990, 52, 741.
(15) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S.
Inorg. Chem. 2001, 40, 4053.
(16) Yang, Q.-Z.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg.
Chem. 2002, 41, 5653.

2746 Organometallics, Vol. 27, No. 12, 2008
Shao et al.
Table 1. Photophysical Parameters of Complexes 1-5 at Room Temperature
Complex
λ
abs/nm (ε/M-1 · cm-1 d
)
λ
em/nm (τem/ns);d Φem
τ
TA/nse
1a
277 (26 400), 330 (16 600), 408 (7550), 428 (6350)
271 (21 300), 300 (25 800), 313 (22 700), 335 (22 400), 406 (11 700),
426 (9100)
285 (21 400), 303 (20 900), 342 (15 400), 423 (6250), 455 (6650)
283 (24 600), 303 (22 800), 339 (16 200), 422 (6710), 448 (6630)
250 (36 700), 260 (36 000), 290 (31 200), 328 (20 300), 424 (6100), 467
(4900)
2b
460
3c
4c
5b
614 (183);0.0007
616 (178);0.0018
460, 622
224
177
a DMF was used as solvent. b Acetonitrile was used as solvent. c CH2Cl2 was used as solvent. d Measured at a concentration of ∼10-5 M.
e Monitored at 705 nm.
Figure 3. Electronic absorption spectra of complexes 1, 3, and 5
in CH2Cl2 and CH3CN (5 is in acetone).
Figure 4. Normalized emission and excitation spectra of ligand L
As a result, the π system of the terdentate ligand is extended,
which further reduces the energy of the π* orbital.
and complexes 2 and 5 in degassed acetonitrile solutions at a
concentration of ∼2.1 × 10-5 M. The excitation wavelength is
325 nm for the emission measurement. The excitation spectra were
monitored at the corresponding emission band maximum.
The solvent-dependent study in CH2Cl2 and CH3CN (acetone
was used for 5 due to its poor solubility in CH3CN) shows that
the 1MLCT bands red-shift in the less polar solvent CH2Cl2 in
comparison to the shifts in CH3CN or acetone (Figure 3),
suggesting that the dipole moment of the ground state is larger
than that of the excited state.8a,17 This is in line with what is
found for platinum terpyridyl complexes.3b When the absorption
spectra of complexes 1 and 2 and those of 3 and 4 are compared,
it is found that there is no significant change within each pair
of complexes. This indicates that the different counteranions
exhibit a negligible effect on the electronic absorption spectra
of these platinum(II) 2,4-di(2′-pyridyl)-6-(p-tolyl)-1,3,5-triazine
complexes.
spectra of 2 and 5 monitored at an emission wavelength of 460
nm were measured. In addition, the emission of the ligand L
upon excitation at 325 nm was monitored. As displayed in
Figure 4, the emission bands at 460 nm for 2 and 5 are quite
similar to that of L, with only an approximately 20 nm red shift.
This red shift could be attributed to the reduced electron density
of the terdentate ligand in the platinum complexes, which would
decrease the energy of the ligand π* orbital and thus induce
the red shift. The attribution of the 460 nm emission band to
1
the π,π* excited-state is further supported by the excitation
spectra measurement. When monitored at 460 nm, the excita-
tion bands of 2 and 5 appear at ca. 318 nm, while the excitation
band of L occurs at 332 nm. Both of them are located at the
ligand 1π,π* absorption band. Therefore, we conclude that the
460 nm emission indeed arises from the ligand π,π* excited
state.
Emission. Complexes 2-5 exhibit weak emission at room
temperature in fluid solutions, while complex 1 shows no
detectable emission in degassed acetonitrile solution. When
excited at 325 nm, both 2 and 5 emit at 460 nm in degassed
acetonitrile solution (shown in Figure 4), and the emission
lifetime is too short to be measured using our instrument, which
1
3
In addition, the emission band at 460 nm for 2 and 5 shows
concentration dependence. As shown in Figure 5 for complex
2, when the concentration increases from 4.0 × 10-6 to 2.1 ×
10-5 M, the emission peak red-shifts and the emission intensity
increases. Above 2.1 × 10-5 M, the emission intensity
decreases. For complex 5 (Figure 6), a similar trend was
observed. Considering the significant ground-state absorption
of 2 and 5 around 450 nm, the red shift and decreased intensity
of the emission band at higher concentrations could possibly
be attributed to an inner-filter effect. On the other hand, either
ground-state aggregation or self-quenching can contribute to a
decrease in the emission intensity. The attribution to ground-
state aggregation could be excluded on the basis of the fact
that no ground-state aggregation was observed from UV-vis
measurement at the same concentration range used for the
emission measurement. However, the possibility of self-quench-
has a time resolution of 5 ns. In comparison to the MLCT
emission of the reported platinum(II) 4′-tolylterpyridyl chloride
and acetylide complexes,3c,d,8a the emission bands at 460 nm
for 2 and 5 are blue-shifted ∼90 and ∼140 nm, respectively,
and the lifetime of the 460 nm band is much shorter than that
3
of the MLCT band of their corresponding 4′-tolylterpyridyl
platinum(II) complexes, which are tens of nanoseconds to
hundreds of nanoseconds, respectively. Therefore, the 460 nm
emission band must not originate from the 3MLCT excited state.
It is very likely that this emission arises from the ligand 1π,π*
excited state, considering the fact that the 325 nm excitation is
1
located at the π,π* absorption band of the ligand. In order to
reveal the origin of this emission band at 460 nm, the excitation
(17) Aldridge, T. K.; Stacey, E. M.; McMillin, D. R. Inorg. Chem. 1994,
33, 722.

Platinum(II) 1,3,5-Triazine Complexes
Organometallics, Vol. 27, No. 12, 2008 2747
Figure 7. Emission spectra of complexes 3 and 4 in degassed
CH2Cl2 solutions at a concentration of ∼2.0 × 10-5 M. The
excitation wavelength is 453 nm.
Figure 5. Emission spectra of complex 2 at different concentrations
in degassed acetonitrile solutions. The excitation wavelength is 325
nm.
Figure 8. Emission spectra of complex 3 at different concentrations
in degassed CH2Cl2 solutions. The excitation wavelength is 453
nm.
Figure 6. Emission spectra of complex 5 at different concentrations
in degassed acetonitrile solutions. The excitation wavelength is 325
nm.
ing still could not be ruled out, because of the time-resolution
limit of our instrument, which prevents us from obtaining the
emission lifetime of the 460 nm emissions.
To verify the contribution from the inner-filter effect, a 5
mm cuvette was used instead of the 1 cm cuvette for the
concentration-dependent emission measurement of complex 2.
When the concentration was increased from 2.0 × 10-5 to 1.0
× 10-4 mol L-1, no obvious bathochromic shift of the 460 nm
emission band was observed. In addition, the quenching of the
emission intensity was also reduced. When the 5 mm cuvette
was used, the emission intensity decreased to 55% in the 1.0 ×
10-4 mol L-1 solution, in comparison to that in the 2.0 × 10-5
mol L-1 solution. In contrast, the emission intensity was
quenched to 12% when measured in the 1 cm cuvette. Therefore,
contribution from the inner-filter effect is clearly evidenced.
For complexes 3 and 4, the emission band appears at ca. 615
nm with a lifetime of approximately 180 ns at room temperature
Figure 9. Emission spectra of complex 5 at different concentrations
in degassed acetonitrile solutions. The excitation wavelength is 479
nm.
1
when excited either within the π,π* transition band (i.e., 324
nm) or within the 1MLCT band (i.e., 453 nm) in CH2Cl2
solutions (shown in Figure 7). In view of the energy and the
lifetime of this emission band and with reference to the emission
energy of other reported platinum terpyridyl complexes,3b,8a,14,16
this emission could be assigned to the 3MLCT excited state. In
comparison to the emission of their 4′-tolylterpyridyl analogue,
the emission bands of 3 and 4 are red-shifted 34 and 36 nm,
respectively.16 Similar to the red shift of the 1MLCT absorption
band, the red shift of the emission band could also be attributed
to the same reason: i.e., the reduced energy of the π* orbital of
the triazine-containing terdentate ligand yields a narrower band
gap between the Pt-based HOMO and the terdentate ligand-
based LUMO, which in turn decreases the energy of the 3MLCT
excited state. Variation of the counteranion introduces a
negligible effect on the emission energies of 3 and 4. However,
the emission quantum yield of 4 is 2.6 times as high as that of

2748 Organometallics, Vol. 27, No. 12, 2008
Shao et al.
Figure 10. Time-resolved triplet transient difference absorption spectra of 3 and 4 in argon-degassed CH2Cl2 solutions at room temperature
following 355 nm excitation. The path length of the cuvette is 1 cm, and the concentration used is 6.2 × 10-5 mol L-1 (A355 ) 0.888) for
3 and 3.9 × 10-5 mol L-1 (A355 ) 0.564) for 4.
3. Nevertheless, both complexes exhibit much weaker emission
than their platinum(II) terpyridyl analogue, which exhibits Φem
) 0.25 in CH2Cl2.16 The reduced emission quantum yields and
lifetimes of 3 and 4 should be related to their lower-lying
emitting states, which are approximately red-shifted ∼35 nm
compared to that of their platinum(II) terpyridyl analogue (580
nm).16 According to the energy gap law,18 when the energy of
the emitting state decreases, the probability of radiationless
decay increases, which in turn reduces the emission quantum
yield and the lifetime.
Triplet Transient Difference Absorption (TA) and Non-
linear Transmission. It has been reported that many platinu-
m(II) terpyridyl complexes exhibit a broad-band triplet excited-
state absorption in the visible extending to the near-IR region,
which usually arises from the 3MLCT state.3 To reveal whether
these 2,4-di(2′-pyridyl)-6-(p-tolyl)-1,3,5-triazine-based com-
plexes also exhibit triplet excited-state absorption, the triplet
transient difference absorption spectra of 1-5 have been
measured. Except for 3 and 4, the other three complexes exhibit
no detectable transient absorption, which presumably is associ-
ated with their short-lived 3MLCT excited state. Figure 10
illustrates the time-resolved transient difference absorption
spectra in argon-degassed CH2Cl2 solutions for 3 and 4. Similar
to the case for their platinum(II) terpyridyl analogue,3c both
complexes exhibit a broad positive absorption band from 510
to 820 nm, indicating that the excited-state absorptions of 3
and 4 are stronger than those of their ground states. The
bleaching bands that occur between 400 and 500 nm are
The concentration-dependent emission of 3 has also been
studied. As shown is Figure 8, when the concentration increases,
the emission band maximum does not change and the emission
intensity continuously increases. This is different from the 1π,π*
emission band at 460 nm for 2 and 5. The difference could
simply arise from the quite different ground-state absorptions
at 460 and 615 nm that eliminate the inner-filter effect.
When excited at its 1MLCT band, for instance 479 nm,
complex 5 exhibits a weak emission at ca. 620 nm, which can
be assigned to the 3MLCT emission and displays a red shift of
approximately 20 nm in comparison to its 4′-tolylterpyridyl
analogue.3b However, the lifetime of this emission was unable
to be obtained, due to the very weak emission. The concentra-
tion-dependent emission spectra of 5 displayed in Figure 9
shows that, at a concentration of 2.7 × 10-4 M, the intensity
of the emission decreases, accompanied by a slight red shift.
Considering the absence of a significant ground-state absorption
around 620 nm and the lack of ground-state aggregation, as
evidenced by a concentration-dependent UV-vis study, the
decreased emission intensity at high concentration likely
originates from self-quenching that is commonly seen in many
organometallic complexes, including platinum complexes.19 For
complex 2, no detectable emission was monitored when it was
excited at its 1MLCT band, i.e. 406 nm, which is similar to the
case for its platinum terpyridyl chloride analogues6a,8a and could
1
consistent with the MLCT bands in the UV-vis absorption
spectra, suggesting that the transient absorption possibly origi-
nates from the 3MLCT excited state. This speculation could be
supported by the lifetime measurement of the transients, which
gives rise to a lifetime of 224 ns monitored at 705 nm for 3
and 177 ns for 4. In comparison to the similarly short lifetime
3
of the MLCT emission (183 ns for 3 and 178 ns for 4), the
transient absorption could arise from the same excited state that
emits or a state that is in equilibrium with the emitting state.
Therefore, the transient absorption could be tentatively assigned
3
to the MLCT state.
Using the partial saturation method,13 the triplet extinction
coefficients of 3 and 4 at 705 nm and the quantum yields of the
triplet excited-state formation have been determined. These
values are ꢀT(705 nm) ) 3400 M-1 cm-1 and ΦT ) 0.21 for 3
and ꢀT(705 nm) ) 690 M-1 cm-1 and ΦT ) 0.83 for 4. Unlike
the effect of the counteranion on the ground-state absorption,
the counteranion affects the triplet excited-state characteristics
drastically, which has also been demonstrated by the emission
quantum yield discussed in the emission section.
3
be ascribed to the quenching of the MLCT state by the low-
lying nonemissive d-d excited state.
As reported earlier by our group, several platinum terpyridyl
based complexes exhibit moderate to strong nonlinear transmis-
sion at 532 nm for nanosecond laser pulses, and the nonlinear
transmission mainly arises from reverse saturable absorption.3
The fact that complexes 3 and 4 exhibit a broad positive band
from 510 to 820 nm implies that the triplet excited-state
(18) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952.
(19) (a) Kunkely, H.; Vogler, A. J. Am. Chem. Soc. 1990, 112, 5625.
(b) Wan, K.-T.; Che, C.-M.; Cho, K.-C. J. Chem. Soc., Dalton Trans. 1991,
1077. (c) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.;
Lipa, D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447. (d)
Lai, S.-W.; Chan, M. C.-W.; Cheung, T.-C.; Peng, S.-M.; Che, C.-M. Inorg.
Chem. 1999, 38, 4046.

Platinum(II) 1,3,5-Triazine Complexes
Organometallics, Vol. 27, No. 12, 2008 2749
∆OD ) (ꢀT - ꢀg)cTl
(2)
where ꢀT and ꢀg are the triplet excited-state and ground-state
extinction coefficients, respectively, cT is the triplet excited-
state concentration, and l is the optical path length of the sample.
For most platinum(II) complexes, the aforementioned as-
sumption is usually valid due to the heavy-atom effect of
platinum, which rapidly depopulates the singlet excited-state
to either the ground-state or to the triplet excited state. Therefore,
532
it is valid to use eq 2 to estimate the εT values for 3 and 4.
Since the ꢀT value at 705 nm has been measured by the partial
saturation method, as discussed in previous paragraphs, the ꢀg
value can be deduced from the UV-vis measurement, ∆OD
can be obtained from the transient difference absorption
spectrum, and the path length for the sample is 1 cm, the triplet
excited-state concentration cT can thus be estimated. Using this
value and the ∆OD and ꢀg values obtained from Figures 10
and 2 at 532 nm, respectively, the ꢀT values at 532 nm are
estimated to be 3130 M-1 cm-1 for 3 and 1690 M-1 cm-1 for
4. These values correspond to σT values of 1.20 × 10-17 cm2
for 3 and 6.47 × 10-18 cm2 for 4 using the conversion equation
σ ) 2303ε/NA, where NA is Avogadro’s constant. When these
values are compared to the ground-state absorption cross-
sections at 532 nm, which are 4.61 × 10-18 cm2 for 3 and 5.25
× 10-18 cm2 for 4, the ratios σT/σg are found to be 2.60 for 3
and 1.23 for 4. Although there could be some extent of
inaccuracy in the determination of the ꢀT values due to the
assumed conditions, the ratio σT/σg for 3 is obviously larger
than that for 4. This probably is the major reason that accounts
for its stronger nonlinear transmission at 532 nm in comparison
to that of 4.
Figure 11. Nonlinear transmission curves of 3-5 for 4.1 ns laser
pulses at 532 nm in a 2 mm cell. The linear transmission of 3-5
was adjusted to 70%. 3 and 4 were dissolved in CH2Cl2, and 5
was dissolved in DMF.
absorption in this spectral range is stronger than that of the
ground state. Therefore, reverse saturable absorption and
nonlinear transmission are expected to occur in this region. To
demonstrate this, nonlinear transmission measurement was
conducted for 1-5 at 532 nm using 4.1 ns laser pulses. To
compare the degree of nonlinear transmission under the same
excitation conditions, all sample solutions except 1 and 2 were
adjusted to have a linear transmission of 70% at 532 nm in a 2
mm cuvette. The linear transmission of 1 and 2 was adjusted
to 90%, due to the limited solubility of these two samples. It is
found that with the increased incident fluence from 0.2 to 3
J/cm2, only complexes 3-5 show a weak transmission drop,
among which 3 exhibits the strongest nonlinear transmission
(shown in Figure 11). No nonlinear transmission was observed
for 1 and 2. In contrast to their corresponding platinum terpyridyl
analogues,3 the nonlinear transmission behaviors of 3-5 are
much weaker. As discovered from other platinum terdentate
complexes, the degree of nonlinear transmission is mainly
determined by the ratio of the excited-state absorption cross-
section (σex) to that of the ground state (σg). Although 3 and 4
exhibit a broad and moderate triplet transient absorption in the
visible to the near-IR region, their ground-state absorption cross-
sections at 532 nm, which are 4.61 × 10-18 cm2 for 3 and 5.25
× 10-18 cm2 for 4, are almost 1 order of magnitude higher
than for the corresponding platinum terpyridyl based complex
(5.31 × 10-19 cm2).3c This would significantly reduce the ratio
σex/σg. As a result, the nonlinear transmission of these complexes
drastically decreases. For complex 5, its ground-state absorption
cross-section is even larger (1.28 × 10-17 cm2); meanwhile,
its triplet excited-state absorption is too weak to be measured.
The combination of these two factors results in a much weaker
nonlinear transmission.
Conclusion
The newly synthesized platinum(II) complexes based on the
2,4-di(2′-pyridyl)-6-(p-tolyl)-1,3,5-triazine ligand exhibit ba-
thochromic shifts in their UV-vis absorption spectra and their
3MLCT emission spectra, which is the result of the electron
deficiency of the triazine ring and the extension of the π
conjugation between the terdentate ligand and the 6-(p-tolyl)
substituent due to the increased coplanarity between the ligand
and the substituent. However, both the emission efficiency and
the lifetime of the emission are significantly reduced compared
to those of their corresponding platinum(II) terpyridyl com-
plexes. Complexes 3 and 4 exhibit broad, moderate triplet
excited-state absorptions from 510 to 820 nm. However, due
to their increased ground-state absorption at 532 nm, their
nonlinear transmission for nanosecond laser pulses is signifi-
cantly reduced. In addition, it has been found that the counter-
anion plays a negligible role in the singlet MLCT state but
exhibits a drastic effect on the triplet MLCT state, as demon-
strated by the emission efficiency, the triplet extinction coef-
ficient, and the triplet quantum yield of 3 and 4. It also affects
the relative nonlinear transmission of these two complexes.
To rationalize the different nonlinear transmissions of 3 and
4, it is important to determine the triplet excited-state absorption
cross-section at 532 nm. In the transient absorption measure-
ments, if all the molecules pumped out of the ground state either
return to the ground state or populate the first triplet excited
Acknowledgment is made to the National Science Foun-
dation (CAREER No. CHE-0449598) and Army Research
Laboratory (No. W911NF-06-2-0032) for support. We are
also grateful to the North Dakota State EPSCoR (ND
EPSCoR Instrumentation Award) for support.
state within the observation time of the optical density change,
532 13a
the following equation can be used to estimate εT
:
OM701285C